Human mothers would never dream of lacing their nurseries with a hearty dose of nicotine, but birds have no such qualms. New research shows that feathery females eagerly collect and line their nests with used cigarettes. The secondhand butts act to repel blood-sucking parasites, demonstrating a rare case in which cigarettes may be the lesser or two evils.

“We know that these materials have an effect of reducing parasites,” said Constantino Macías Garcia, an animal behavior expert at the National Autonomous University of Mexico and co-author of a paper in Biology Lettersdescribing the finding. “Smoked cigarette butts make the mites uncomfortable,” he said. “They’re not happy, so they move away.”

Birds such as starlings and chickadees living in more rural environments often collect volatile plants, such as dandelions and species from the Nicotiana genus, to line their nests with. The plants’ smelly compounds repel parasites, which feed off of adults’ and chicks’ blood, skin and feathers and build up in nests over the breeding season. Enough nest parasites can detrimentally impact a chick’s health or even threaten its survival.

Garcia first noticed an abundance of cigarette materials in nests his students collected from around their Mexico City campus. He wondered if urban birds were substituting cigarettes in lieu of parasite-repelling plants they normally would gather from forests or fields, and decided to test this hunch.

Garcia and his colleagues placed thermal traps, or small strips of metal heated with batteries and coated with sticky tape, in more than 50 house sparrow and house finch nests around the university. Parasites seek out their victims using body heat cues, so the mites naturally congregate towards the warm traps. To smoke out the possible reasons behind the cigarettes’ presence, Garcia lined either side of the traps with fiber from used or unused Malboro cigarette butts.

Traps with the smoked butts, he found, collected significantly fewer parasites than those with the unsmoked butts or those with no butts at all that served as control nests. “It’s likely that they recognize the presence of chemical substances that could be harmful for them, so they don’t approach that area,” Garcia said.

Whether mother birds recognize this property or not, however, remains an unknown. In addition to containing nicotine and other mite-mitigating chemicals, used cigarette filters are fluffy and soft, so birds could be picking them up for their insulating rather than pesticidal properties. Garcia plans to tease out this association with further experiments during next spring’s breeding season, for example, by allowing birds to chose either smoked or unsmoked cigarette butts, or by manipulating the number of parasites in the nest to see if birds still add cigarettes to parasite-free abodes.

Mexico City, of course, is not the only urban center full of discarded cigarettes. Garcia expects that nests in other urban centers, from Beijing to New York to London, would also contain their fair share of cigarette butts. “I hope we’re not giving the idea that we have a particularly bad university full of lots of rubbish,” he said. “I’d be very surprised if this was not something going on in other places around the world.”

While birds seem to have found a way to reduce their parasite load (whether intentionally or not), the study is not necessarily all good news for avians. Cigarettes contain many chemicals, quite a few of which are toxic. Agrochemicals such as pesticides may wind up in cigarettes during production, which, as Rachel Carson and countless environmentalists have pointed out, tend to be bad news for birds. Mother birds sit atop nests for long periods of time, giving any residual chemicals ample occasion to find a contamination path, and chicks spend the most vulnerable period of their lives in this potentially hazardous nursery. Garcia intends to conduct studies investigating impacts the cigarettes may be having on the adult or baby birds.

Finally, while it’s encouraging to know that birds can adapt to our pollution-filled urban landscape, all species may not be equal in their flexibility. Some groups of birds may prefer traditional nest-building materials, excluding them from using all the bits of trash that wind up on city streets around the world.

“We may be causing asymmetries in how birds respond to the environment, favoring some and not others,” Garcia said. “We can’t necessarily conclude that we can do whatever with the environment and nature will just solve the problem for birds.”

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